The present invention relates generally to the construction of replacement filter cartridges and, more particularly, to a method and apparatus for making a filter support tube for use in a filter cartridge.
At the heart of all replacement filter cartridges is a means of structural mechanical support. For four decades or more, a perforated metal support tube has been widely used as a structural support member for all types of pleated paper or other types of filter cartridges and also for coalescers to provide mechanical strength against rupture from the "outside in" direction or from the "inside out" direction. Also, the perforated metal support tube provides a means of creating longitudinal support for a gasket seal.
There have been many improvements in the perforated metal support tube for filter cartridges over the years but the basic design of this structure has remained the same. Basically, it is a relatively thin-walled metal cylinder perforated with small holes over its total surface area. This design has several fundamental weaknesses which has rendered it far from the ideal as a structural strength member.
One fundamental weakness is that the "open area", in other words the perforations or holes, provided in the cylinder wall to allow adequate fluid flow affects the structural strength of the tube. To provide the barest minimum of structural strength of the tube it is necessary to limit perforations to only about 35% to 40% of the total surface area of the cylinder wall. This means that about 60% of the surface area of the cylinder wall is completely blocked to fluid flow. It is readily apparent that this restriction to fluid flow is extremely detrimental to filter performance in that the filter life is decreased, initial and final differentials are much higher, a loss in filter efficiency is incurred. Further, this restricted flow area becomes particularly significant in the case of air or gas filtration where the pressure differential across the perforated metal cylinder is frequently greater than the pressure differential across the filter media itself.
Another fundamental weakness is that manufacturing of the perforated metal support tube for a filter element requires that a relatively thin perforated metal be used in order to permit easy formation of the cylindrical shape of the tube. However, such thin perforated metal has very low strength to resist crushing when pressure differentials build up on the filter media in the "outside in" direction especially. Improvements have been made by corrugating the metal to provide ridges which add to structural strength. Also, perforated metal cylinders are now available which include a helical lock seam which greatly strengthens the structural member but further complicates the "blinding" of the available surface area. Frequently, it is necessary to reinforce the perforated metal cylinders with a spiral helical wire which is spot welded to the perforated metal to provide additional structural strength. The helical lock seam also tends to collapse longitudinally when trying to effect a gasket seal.
Still another fundamental weakness is the use of small perforations or holes which adversely affects filter efficiency. The small holes (for instance 1/8 to 1/4 inch diameter) in the perforated metal cylinder causes localized high velocities in fluid flow which tends to drive small particles of one to two microns in size through the filter media by the force of momentum (1/2 mv.sup.2). This is particularly significant in the case of coalescer elements where the coalescer performance specifications require Red Iron Oxide as the basic solids contaminant with greater than 94% of these particles being less than one micron in size.
Consequently, a need still exists for an alternative filter support tube design that will overcome the weaknesses described above as inherent with the prior art perforated metal design. Also a need exists to provide the machinery necessary to automatically make such alternative tube design.